In a CDMA (Code Division Multiple Access)-based mobile communication, the amplitude and phase of an information symbol fluctuate under the influence of Raleigh fading.
A “coherent detection system using a pilot symbol” is proposed as a method for carrying out coherent detection by compensating the phase of information symbols on the receiving side.
The coherent detection system using a pilot symbol is a method for carrying out coherent detection by estimating fluctuations in the phase of information symbols using pilot symbols (pilot bits) whose phase is known periodically (for example, every 1.25 ms) inserted in an information signal and compensating the phase of the information symbols through interpolation.
More specifically, the coherent detection system using a pilot symbol repeats an operation consisting of comparing the phase of a demodulated pilot symbol with an ideal phase (original phase) of the pilot symbol and calculating the phase error over a plurality of slots, and determines the phase of the information symbol between neighboring pilot symbols from the phase error calculation result through interpolation and thereby carries out coherent detection.
The coherent detection system using a pilot symbol is described, for example, in “Wideband CDMA Open-Air Transmission Result (RCS97-3)”.
FIG. 6 shows a configuration example of a reception signal.
A plurality of pilot symbols (pilot bits) 140, 150, 160 and 170 in FIG. 6 are known symbols inserted for every slot.
Detection target slot 2001 is a 1-slot information symbol to be detected. To compensate the phase of this 1-slot information symbol, the amount of phase fluctuation is estimated using pilot symbols over a few slots before and after the detection target slot.
For example, suppose a signal in the configuration shown in FIG. 6 is being received continuously. Suppose a case where coherent detection is performed using a plurality of pilot symbols 140 to 170. In this case, the information symbol which is the target of coherent detection is information symbol 2001 (that is, the detection target slot), two slots ahead of slot 2003 currently being received.
Thus, it is necessary to temporarily store the previously received information symbols (information symbols up to the one two slots ahead) until the estimation of phase fluctuation using pilot signals (pilot symbols) over a plurality of slots is completed.
That is, a coherent detection circuit requires a memory capable of storing symbols corresponding to at least 3 slots.
On the other hand, a CDMA communication system allows multi-code transmission to speed up communications of large volume image data, etc.
Multi-code transmission is a data transmission system intended to effectively use code resources by changing the number of codes to be multiplexed according to the amount of data.
For example, when sound data or a small-volume packet is transmitted, data is transmitted with one spreading code assigned to one physical channel.
On the other hand, when large-volume data such as a motion picture is sent, the motion picture data is divided into a plurality of physical channels, each physical channel is assigned one spreading code and those divided physical channels are multiplexed to carry out data transmission.
Multi-code transmission does not always use all codes, but dynamically determines codes to be used according to the situation. However, it is necessary to secure the memory area necessary to carry out the above-described coherent detection using a pilot symbol assuming the case where all channels are used.
There are various ways of memory configurations and it goes without saying that it is advantageous in every aspect of the area occupied, power consumption and ease of control, etc. to provide a common memory and control the memory in a concentrated manner rather than providing a plurality of memories and controlling them individually.
However, the following problem may arise in the case where electrically continuous memory areas large enough to store all reception data are provided for when the number of multiplexed channels reaches a maximum in order to be able to respond to dynamic changes in the number of multi-codes.
That is, since the same memory area is accessed regardless of the number of multiplexed codes, the entire memory area becomes the access target even in the case where data is transmitted with one code or where a plurality of codes is multiplexed. As a result, power consumption of the memory circuit always remains unchanged.
Further, in coherent detection using a plurality of pilot symbols over a plurality of slots, memory accesses occur when the information symbol of a slot currently being received is written and the information symbol of a detection target slot is read. That is, it is not necessary to generate memory accesses for other slots.
However, in the symbol storage memory configuration of a conventional coherent detection circuit, memory areas are electrically continuous, and therefore even the memory area which need not be accessed may also be accessed and power is wasted accordingly.
The present invention is implemented based on such consideration and it is an object of the present invention to attain low power consumption in a memory circuit and coherent detection circuit.